The present invention relates to nondestructive testing techniques, and more particularly to the calibration of eddy current testing equipment for measuring the profile of a tube or the like.
In numerous industrial installations, and particularly nuclear power plants, a variety of different types of tubes are configured for the purpose of carrying steam or process fluid, or for serving as a sheath or sleeve for guiding the movement of a rod. Due to mechanical, thermal, or other stresses, such tubes may experience slight deformation which, if not identified quickly, can lead to significant bending and adversely affect the performance of the tube and even the entire industrial process.
Conventionally, eddy current inspection techniques have been used to assess the degree to which the circularity of a tube cross section has deteriorated. Typically, an eddy current probe is advanced through the tube at a known rate, and data are collected from for example, eight eddy current sensors surrounding the probe. The resulting data must then be analyzed to determine tube circularity as a function of axial location along the tube.
Equipment of this type includes the Zetec MIZ-18 System, available from the Zetec Company. The Zetec and similar equipment must be properly calibrated in order for the acquired data to provide accurate results regarding the tube profile. The probe output signal is typically non-linear with respect to changes in the tube radius. Conventionally, a step tube is used as a calibration standard associated with a particular inspection application. For example, if the tubes of a nuclear steam generator are to be inspected, the material, nominal ID of the tubes and probe body diameter dictate the type of step tube used for calibration. For a steam generator tube of Inconnel material with a nominal ID of 0.654 inch, a step tube of the same material has two regions, one with an ID of 0.654 inch and another with an ID of, for example 0.577 inch.
The equipment operator obtains probe output readings (hereinafter referred to as eddy current test units, or "ECTU") for the nominal and step regions of the step tube standard. These inspection data are acquired from the steam generator tubes, as a function of probe axial position in the tubes. Conventionally, the step tube readings are used to look up tabular calibration correction values for the ECTU of the steam generator tubes. This look up can be manual, or the "look up table" can be in the form of a data base in the memory of a computer.